The invention relates to a burn-in gumming composition for irradiated and developed offset printing plates.
Offset printing plates in general comprise: (a) a two-dimensional carrier material of metals and/or plastics, the surface of which can be modified, for example, by roughening (etching) and/or anodic oxidation, and (b) at least one radiation-sensitive (photosensitive) coating (reproduction layer). Particularly in the case of positive-working reproduction layers, but
in principle also with negative-working reproduction layers, the preparation of a printing form with the basic process steps of irradiation, development and gumming (preserving, desensitizing) also includes a burn-in step between the last two steps. Applied to positive-working reproduction layers, the procedure can be illustrated as follows. During the imagewise irradiation (exposure), those parts of the reproduction layer which are struck by electromagnetic energy (light) become soluble, and the unaffected parts remain insoluble (in each case relative to the developer to be used). As a result of the first-mentioned parts being dissolved out in the developing step, the non-image areas, which are water-bearing during the later printing from the printing form, are formed on the surface of the printing plate after the development of the irradiated reproduction layer, and the image areas which are inkbearing during later printing are formed from the lastmentioned parts. In practice, it has been found that heating of the irradiated and developed printing plate -depending on the nature of the carrier material and/or the production layer - to more than 180.degree. C can lead to longer print runs with the printing form thus resulting. This process is called "burning-in". The burning-in achieves a consolidation of the image areas, the latter becoming, for example, insoluble in the usual organic solvents and resistant to the action of chemicals.
During burning-in, constituents of the image areas (i.e., of the reproduction layer), for example, constituents of the binder, can also pass into the nonimage areas and hence adversely affect the hydrophilic character of the latter, leading, for example, to toning (absorption of printing ink in the non-image areas). Therefore, it was necessary, in the initial phase of using a burning-in step, to remove the impurities in the non-image areas again by means of relatively aggressive solutions.
In the course of time, so-called "burning-in aids" were developed which at least facilitated this after-cleaning. However, as a rule, after-cleaning nevertheless still required cleaning of the non-image areas with water before the actual printing process. The burning-in aids known from the state of the art include, inter alia, the following examples.
German Patent No. 2,318,286 (corresponding to U.S. Pat. No. 3,745,011) discloses a process for the preparation of negative resist images, wherein a photoresist is treated, after exposure and development and before a burn-in process, with a solution of a cycloaliphatic, heterocyclic, or aromatic polycarboxylic acid or an aromatic sulfonic acid. The photosensitive layers of the photoresists used contain natural proteins, such as fish glue, and ammonium bichromate. This treatment before burning-in is intended to: (a) lower the burning-in temperature; (b) increase the etch resistance of the resist image; and (c) improve the stripoability of the resist image or the applied photoresist. The acids used include, inter alia, p-toluenesulfonic acid, 2-napthalenesulfonic acid, naphthalene-1,8-disulfonic acid, benzenesulfonic acid, and 1,3-benzenedisulfonic acid, which are employed preferably in the form of a 1 to 25 % solution. Steel is used exclusively as the carrier material for the coating with photoresist. The application for printing plates and the special problems with burning-in in that case are not mentioned.
German Patent No. 2,530,422 (corresponding to British Patent Specification No. 1,513,368) discloses a process for the preparation of printing forms, plates for printed circuits, integrated circuits and the like, wherein the exposed and developed carriers are, before burning-in, provided with a protective layer at least on the non-image areas. This protective layer remains on the carrier during burning-in and can, after burning-in, be removed by means of water together with a precipitate which settles on the protective layer during burning-in. The protective layer is applied in the form of an about 2.5 to 45% aqueous solution of one of the following compounds: sodium dodecylphenoxybenzenedisulfonate, a sodium salt of an alkylated naphthalenesulfonic acid, disodium methylenedinaphthalenedisulfonate, sodium dodecylbenzenesulfonate, a sodium salt of a sulfonated alkyldiphenyl oxide, an ammonium perfluoroalkylsulfonate, a potassium perfluoroalkylsulfonate, sodium dioctylsulfosuccinate, sodium di-(methylamyl)-sulfosuccinate, and lithium nitrate.
In British Patent Specification No. 1,575,200 a water-soluble hydrocolloid from the group comprising gum arabic, sodium carboxymethylcellulose, or alginate is also added to an aqueous solution for producing the protective layer before burning-in, in addition to one of the compounds listed above. The mixture of substances used is intended to comprise 85 to 98% by volume of one of the compounds known from British Patent Specification No. 1,513,368, and 2 to 15% by volume of one of the hydrocolloids, and it is employed in a 10 to 50% by volume aqueous solution. After burning-in, the printing forms are washed with water and can then be clamped into a printing press.
The burn-in aid according to German Patent No. 2,625,336 (corresponding to U.S. Pat. No. 4,063,507) is boric acid or a borate and is employed in solid form or preferably in a 3 to 10% aqueous solution.
According to German Offenlegungsschrift No. 2,626,473 (corresponding to British Patent Specification No. 1,555,233), it is possible, in place of the compounds already described above as suitable for the burning-in of printing plates (burn-in aids), also to use, inter alia, various hydrophilic polymers such as gum arabic, dextrin, polyvinyl alcohol, cellulose ethers, homopolymers and copolymers based on acrylic or methacrylic acid or a copolymer of alkyl acrylate and vinylmethylacetamide or also salts of organic acids such as di-sodium anthraquinone-2,7-disulfonate, tri-sodium naphathalene1,3,5-trisulfonate, tri-sodium 1-naphthylamine-4,6,8-trisulfonate, or tri-sodium 1-naphthol-3,6,8-trisulfonate. The compounds are used in aqueous solution in a concentration from 0.1% up to saturation.
European Patent Application No. 0,043,991 (corresponding to U.S. Pat. No. 4,355,096) lists amines containing carboxyl groups, such as ethylenediaminetetraacetic acid, nitrilotriacetic acid or salts thereof as burn-in aids, and U.S. Pat. No. 4,191,570 lists salts or salt-like reaction products of naphthalene, formaldehyde and sulfuric acid.
However, all the known burn-in aids still have serious disadvantages. For example, they can effect a reduction in the oleophilic character of the image areas, and they can lead to staining of the printing plate surface and, particularly in the case of insufficient postcleaning with water or aqueous developer solutions, to interference with the water/printing ink balance during the later printing. In operating instructions for the use of many of the known burn-in aids, these problems are specifically pointed out, and it is stated that these problems can be overcome, for example, by dry rubbing of the printing plate surface before burning-in, by careful rinsing or even (for example in European Patent Specification 0,012,956 (corresponding to U.S. Pat. No. 4,265,999)) or by providing hydrophilic properties by posttreatment/cleaning with an aqueous solution of a hydrophilic polymer such as polyvinylphosphonic acid. Such special posttreatments providing hydrophilic properties, or one of the conventional gumming (preserving) post-treatments are possible in principle, but require an additional working step in the preparation and use of a printing form. A combination of the two steps in a burning gumming is described in British Patent Soecification No. 1,575,200, described above, but it still leads to a large consumption of paper in the printing press as, without additional intermediate cleaning, the printing form is too slow to run clean.
In European Patent Application No. 0,155,620, a burn-in gumming is proposed which comprises several components, including a hydrophilic polymer, a water-soluble organic carboxylic or sulfonic acid or one of their water-soluble salts, a hydroxycarboxylic acid and/or salts thereof, an alkanediol and a surfactant. A printing form treated with this burn-in aid need no longer be washed off after burning-in but can be clamped directly into the printing press. In this case, a relatively large number of components is employed, and it can happen that, in the event of unduly thick preservation, as is frequently the case in practice with manual treatment, the printing stencil can be extensively damaged during burning-in, so that perfect printing of long runs is no longer possible.